The invention concerns a flow meter, particularly for liquid fuels in petrol pumps, with at least two inter-engaging screw spindles through which liquid can flow in the axial direction and which are mounted at their ends in a housing so as to rotate, wherein the housing includes a measuring chamber through which liquid can flow and in which an exciter disc, which is rigidly connected to one of the spindle, can rotate in order to cooperate with a detector at the housing end.
A generic flow meter is known from EP 05 72 621. The flow meter disclosed therein is used in petrol pumps for liquid fuels, such as Petrol, Diesel or similar. High levels of accuracy are demanded from such measuring arrangements, the device must be able to be calibrated and, in addition, a device of this type must be relatively inexpensive in manufacture and have a long service life. The measuring device described contains a case, in which two intermeshing screw spindles are arranged. The screw spindles are mounted at each end, so as to rotate with easy action. The outer flanks of the screw spindles rotate very closely to the inner case wall in corresponding bores. From one of the axial directions the fluid to be measured flows into the spindle arrangement where the rate of its flow causes the spindles to rotate. The fluid leaves the spindle arrangement at the opposite axial direction and passes through a measuring chamber, in which an exciter disc, which is rigidly connected to one of the spindles, is allowed to rotate. The exciter disc, rotating in the flow of the fluid, co-operates with a detector at the housing end, which receives information on rotational speed transmitted by the exciter disc and which feeds this to an electronic evaluation circuit to convert the measured pulses into values of volume. From DE 42 08 869 a device is also known to determine the volumetric flow in a fuel line. In that case a steel pole wheel is fitted to the end of a spindle which co-operates with an inductive proximity switch. To enable identification of the direction of rotation of the spindles, two proximity switches are provided and screwed into the housing, opposite but offset to each other, and reaching into the fluid flow within the measuring chamber. The disadvantage of such proximity switches is their complex method of assembly, since they have to be screwed into appropriately tapped holes in the measuring chamber and are then directly positioned within the flameproofed section of the measuring arrangement. In addition, proximity switches require a large cross section of the measuring chamber flow channel.
Correct operation of a generic flow meter assumes, that the spindles are allowed to rotate with a minimum application of force. This requires, that the braking action caused by an exciter disc, rotating within the fluid, is minimised.